The invention described is directed to detectors for detecting the envelope of an electrical signal. The invention is additionally directed to electrical networks for improving the linearity of an electrical signal which can be expressed in terms of a polynomial.
In the past, it has been common practice to detect the envelope of an AM modulated signal by using a diode detector. Such detectors are commonly used as video detectors in television receivers.
A well known problem associated with diode detectors is that their output is generally not as linear as desired when driven by low level signals. To achieve a commonly accepted degree of linearity, the signal which is to be detected by the diode detector is caused to have a relatively large amplitude. In that manner, the diode operates on the more linear portion of its transfer curve. However, in modern television receivers constructed with integrated circuit chips, a high amplitude video signal may not be readily available for application to the diode detector. Hence, increased non-linearity in the detected video signal results.
Further, the degree to which a diode detector develops a linear output depends substantially on the quiescent bias applied to the diode. Hence, that bias level must be maintained very accurately at a predetermined level. Even so, the output of the diode detector is not as linear as is desired. Similar problems are experienced in applications other than television when diode detectors are used.
Another commonly encountered problem, whether it be in connection with a signal detector or not, is that a signal to be processed may contain a component which is desired to be recovered and an undesired component which contributes to non-linearity in the signal. Frequently, such a signal to be processed is expressable in terms of a polynomial. For example, where the composite signal to be processed is expressable in terms of the sum of the squares of a desired component and an undesired component, it is generally desired to process the composite signal so that the resultant signal is equal only to the square root of the square of the desired component of the composite signal. In the past, electrical networks for providing that function have been unduly complex or have not completely eliminated the undesired component of the composite signal. In particular, there has been no satisfactory network for removing undesirable components from the output of signal detectors so as to increase the resultant linearity of their detected signal.